The invention relates to a method for establishing the rotational speed of mechanically commutated d.c. motors and/or the distance traveled by them in accordance with the preamble of Patent claim 1. Such methods are described in, for example, EP 0 689 054 A1, EP 0 730 156 A1 and DE 39 35 585 A1.
It is general knowledge that the alternating component of the current in d.c. motors, the ripple, can be detected, evaluated and used as a measure of the rotational speed of the motor. In DE 35 27 906 A1, for example, a so-called zero-crossing method is described in which the zero crossings of the current are detected after elimination of the direct component.
It is furthermore known that the behavior of d.c. motors can be described by means of an electromechanical motor state model based on the motor equations as shown in FIGS. 4a and 4b. The motor equation U.sub.q (t)=c.multidot..PHI..multidot.n(t), also known as generator equation, the motor equation M.sub.m (t)=c.sub.1 .multidot..PHI..multidot.I.sub.a (t) and the electrical relationship ##EQU1## is also to be found in relevant literature such as, for example, Lindner: Reference Volume "Elektrotechnik--Elektronik", Leipzig, 2nd Printing 1983, p. 199 et seq. The reference symbols (see FIGS. 4a, 4b and also the equations) are selected analogously in an appropriate form and signify specifically U.sub.q the induced armature voltage; c.sub.1, c.sub.2 the motor-specific quantities, also known as motor constants, .PHI. the magnetic flux, n the rotational speed, M.sub.L the load moment, M.sub.m the motor moment and M.sub.B the resulting acceleration moment, I.sub.a the motor or armature current, U.sub.M the motor terminal voltage, R.sub.a the armature resistance, R.sub.k the external terminal resistance, L the inductance of the motor winding and J the mass moment of inertia of the entire rotating arrangement including the parts to be moved such as, for example, the windows.
In the generic method of EP 0 689 054 A1, the generated signal of ripple is phase shifted and the phase-shifted signal is added or subtracted to the non-phase-shifted whereby a signal is obtained that is free of double commutation phenomena.
The so-called ripple-count method is also described in this literature. The paper "Unkonventionelle Drehzahimessung and -regelung bei Gleichstrommotoren" (Unconventional speed measurement and control of d.c. motors) by Birk, Elektronik 25, 14.12.1984, p. 71 et seq., describes the incremental determination of speed from the commutation ripple of the armature current.
The fundamental problems associated with the detection and evaluation of ripple are also mentioned in the above literature, in particular the susceptibility to faults in the line voltage and double commutations. The circuitry involved is considerable in all compensation methods and ultimately the possibility of faults occurring cannot be excluded.